1. Field of the Invention
This invention relates to a method of producing vinyl acetate through reaction of ethylene, oxygen and acetic acid in vapor phase, employing a catalyst which comprises metallic palladiu, metallic gold and an alkali metal compound, all deposited on a suitable porous carrier or support. The catalyst components, the proportions thereof, as well as their distribution curves within the porous support, must all fall within certain well-defined limits.
2. Description of the Prior Art
It is known in the art to produce vinyl acetate by reacting ethylene, oxygen and acetic acid in gaseous phase and in the presence of a catalyst comprising palladium, gold and an alkali metal acetate supported on certain carrier materials. Generally, such catalyst system exhibits high activity, but the reaction proceeds too rapidly resulting in various deficiencies and disadvantages, depending on the distribution pattern or profile of the catalyst components which are deposited on, and in relation to the carrier. More particularly, when use is made of the known catalyst systems comprising a porous carrier impregnated with palladium and/or gold, the metal components deposited at or about the support interiors or central regions do not contribute signficantly to the reaction mechanism, since the reactants are scarcely able to diffuse into the central or inner regions of the porous network of the catalyst and hence, the reaction occurs substantially only at the outermost or surface regions of the catalyst. Therefore, the catalyst components, in those reactions known in the art, in large part do not contribute to the reaction scheme, resulting in a reduction in catalytic efficiency per unit weight of the catalyst components. Also, there is encountered an economic disadvantage in that the catalyst components at the inner regions of the carrier are only difficultly recovered and reused. Furthermore, the use of a highly active catalyst at times gives rise to side reactions and, therefore, leads to a reduced selectivity for the contemplated reaction product.
On the other hand, a catalyst wherein the palladium metal and/or gold metal has not substantially penetrated into the carrier, but rather bears most of the metallic component deposited on the carrier surface only, displays a limited catalyst life, and does not permit of the production of vinyl acetate in high yield.